Layout and page setup

I’m using a script to allow the users to select from different map templates, the templates range in paper size from Letter to ansi D.

My script finds a chosen attribute and zooms the map to that location, then exports to map as a pdf. My problem is that the page/paper size doesn’t appear to change when switching to the target mxd.

I notice the same behavior when using the ‘switch layout’ function in the Layout toolbar.

Any idea how to use the page and paper settings of my target MXD?

Thanks